1. Field of the Invention
The present invention relates to a video telephone and particularly to a video telephone for transmitting and receiving video signals and voice signals through an Integrated Services Digital Network (ISDN). Moreover, the present invention relates to a method for controlling the transmission and reception of signals when a power supply of the video telephone fails.
2. Description of the Prior Art
A basic ISDN interface includes at least two B channels and one D channel. The B channel currently used has a bit rate of 64 kbits/sec and the D channel has a bit rate of 16 kbits/sec. Communication between video telephones is carried out using an ISDN B channel.
A general communication method between video telephones will be explained. A video telephone system is usually provided with a video signal coder/decoder (CODEC) which encodes an input video signal to obtain a signal having a bit rate of 48 kbits/sec or decodes such signal into an image signal. A voice signal CODEC encodes an input voice signal to obtain a signal having a bit rate of 16 kbits/sec or decodes such signal into a voice signal. The data coded by the video signal and voice signal CODECs are multiplexed and the data is then transmitted as data having a bit rate of 64 kbits/sec to a distant video telephone system using the B channel. On the receiving side, the received data is decoded to obtain a voice signal and an image signal. Communication between video telephone systems can be realized through the signal transmission and reception explained above.
An ordinary telephone set is operated by a current supplied through a speed path and receives its power from telephone lines. A video telephone is operated, unlike the ordinary telephone set, by receiving its power from an external power source. The video telephone has high power consumption and the power supplied through telephone lines is insufficient. This is because the video telephone is provided with a monitor, camera and codec which require high speed encoding operations. The video telephone is also provided with a sub-power source which supplies power if the external power source fails.
Since, however, a sub-power source having a remarkably high power capacity is required for normal operation of a video telephone, the video telephone becomes large in size. In contrast, if a sub-power source having a low power capacity is used, and the video telephone does not function normally for a long period of time, the video telephone using the sub-power source cannot provide sufficient communication services.